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What is the value of \(\frac{a^{2}~+~ac}{a^{2}c~-~c^{3}}-\frac{a^{2}~-~c^{2}}{a^{2}c~+~2ac^2~+~c^3}-\frac{2c}{a^2~-~c^2}+\frac{3}{a~+~c}\) ?

This question was previously asked in
CDS I 2020 Elementary Mathematics Previous Year Paper (02-Feb-2020)
The correct answer is \(\frac{6}{a+c}\)

Understanding the Algebraic Expression Simplification

We need to simplify the given algebraic expression. This involves factoring polynomials, finding a common denominator, and combining terms. Let's break down the expression term by term.

The expression is:

\(E = \frac{a^{2}~+~ac}{a^{2}c~-~c^{3}}-\frac{a^{2}~-~c^{2}}{a^{2}c~+~2ac^2~+~c^3}-\frac{2c}{a^2~-~c^2}+\frac{3}{a~+~c}\)

Simplifying Each Term

Let's simplify each fraction in the expression by factoring the numerator and the denominator.

  1. First term: \(\frac{a^{2}~+~ac}{a^{2}c~-~c^{3}}\)
    • Numerator: \(a^{2} + ac = a(a+c)\)
    • Denominator: \(a^{2}c - c^{3} = c(a^{2} - c^{2}) = c(a-c)(a+c)\)
    • Simplified first term: \(\frac{a(a+c)}{c(a-c)(a+c)} = \frac{a}{c(a-c)}\) (assuming \(a+c \neq 0\))
  2. Second term: \(\frac{a^{2}~-~c^{2}}{a^{2}c~+~2ac^2~+~c^3}\)
    • Numerator: \(a^{2} - c^{2} = (a-c)(a+c)\)
    • Denominator: \(a^{2}c + 2ac^{2} + c^{3} = c(a^{2} + 2ac + c^{2}) = c(a+c)^2\)
    • Simplified second term: \(\frac{(a-c)(a+c)}{c(a+c)^2} = \frac{a-c}{c(a+c)}\) (assuming \(a+c \neq 0\))
  3. Third term: \(\frac{2c}{a^2~-~c^2}\)
    • Numerator: \(2c\)
    • Denominator: \(a^2 - c^2 = (a-c)(a+c)\)
    • Simplified third term: \(\frac{2c}{(a-c)(a+c)}\)
  4. Fourth term: \(\frac{3}{a~+~c}\) (already in simplest form)

Substitute the simplified terms back into the expression:

\(E = \frac{a}{c(a-c)} - \frac{a-c}{c(a+c)} - \frac{2c}{(a-c)(a+c)} + \frac{3}{a~+~c}\)

Finding a Common Denominator

The denominators are \(c(a-c)\), \(c(a+c)\), \((a-c)(a+c)\), and \((a+c)\). The least common denominator (LCD) is \(c(a-c)(a+c)\).

Rewrite each term with the LCD:

  • \(\frac{a}{c(a-c)} = \frac{a \cdot (a+c)}{c(a-c) \cdot (a+c)} = \frac{a(a+c)}{c(a-c)(a+c)}\)
  • \(\frac{a-c}{c(a+c)} = \frac{(a-c) \cdot (a-c)}{c(a+c) \cdot (a-c)} = \frac{(a-c)^2}{c(a-c)(a+c)}\)
  • \(\frac{2c}{(a-c)(a+c)} = \frac{2c \cdot c}{(a-c)(a+c) \cdot c} = \frac{2c^2}{c(a-c)(a+c)}\)
  • \(\frac{3}{a+c} = \frac{3 \cdot c(a-c)}{(a+c) \cdot c(a-c)} = \frac{3c(a-c)}{c(a-c)(a+c)}\)

Now, combine the terms with the common denominator:

\(E = \frac{a(a+c) - (a-c)^2 - 2c^2 + 3c(a-c)}{c(a-c)(a+c)}\)

Expanding and Simplifying the Numerator

Expand the terms in the numerator:

  • \(a(a+c) = a^2 + ac\)
  • \((a-c)^2 = a^2 - 2ac + c^2\)
  • \(2c^2\)
  • \(3c(a-c) = 3ac - 3c^2\)

Substitute these back into the numerator expression:

Numerator \( = (a^2 + ac) - (a^2 - 2ac + c^2) - 2c^2 + (3ac - 3c^2)\)

Remove the parentheses, remembering to change signs for the terms inside the second parenthesis (because of the minus sign in front):

Numerator \( = a^2 + ac - a^2 + 2ac - c^2 - 2c^2 + 3ac - 3c^2\)

Group like terms:

Numerator \( = (a^2 - a^2) + (ac + 2ac + 3ac) + (-c^2 - 2c^2 - 3c^2)\)

Combine like terms:

Numerator \( = 0a^2 + 6ac - 6c^2\)

Numerator \( = 6ac - 6c^2\)

Factor the numerator:

Numerator \( = 6c(a-c)\)

Final Simplification

Now substitute the simplified numerator back into the expression:

\(E = \frac{6c(a-c)}{c(a-c)(a+c)}\)

Assuming \(c \neq 0\) and \(a-c \neq 0\), we can cancel out the common factor \(c(a-c)\) from the numerator and the denominator:

\(E = \frac{6}{a+c}\)

This is the simplified value of the expression.

Step Operation Result
1 Factor first term \(\frac{a}{c(a-c)}\)
2 Factor second term \(\frac{a-c}{c(a+c)}\)
3 Factor third term denominator \(\frac{2c}{(a-c)(a+c)}\)
4 Find LCD \(c(a-c)(a+c)\)
5 Rewrite with LCD and combine numerators \(\frac{a(a+c) - (a-c)^2 - 2c^2 + 3c(a-c)}{c(a-c)(a+c)}\)
6 Simplify numerator \(6c(a-c)\)
7 Final simplification \(\frac{6}{a+c}\)

Conclusion

The value of the given algebraic expression is \(\frac{6}{a+c}\).

Revision Table: Key Concepts in Algebraic Simplification

Concept Description Importance
Factoring Polynomials Breaking down a polynomial into simpler expressions (factors) that multiply together to give the original polynomial. E.g., \(a^2 - c^2 = (a-c)(a+c)\). Essential for simplifying fractions and finding common denominators.
Least Common Denominator (LCD) The smallest multiple that is common to all denominators in a set of fractions. For algebraic fractions, it's the product of the highest powers of all prime factors in the denominators. Necessary for adding or subtracting fractions.
Combining Algebraic Fractions To add or subtract fractions, they must have a common denominator. Once they do, combine the numerators over the common denominator. Allows performing addition/subtraction operations on rational expressions.
Simplifying Fractions Canceling common factors from the numerator and denominator. This reduces the fraction to its simplest form. Makes expressions easier to work with and understand.

Additional Information: Working with Rational Expressions

Rational expressions are fractions where the numerator and denominator are polynomials. Simplifying, adding, subtracting, multiplying, and dividing rational expressions follow rules similar to those for numerical fractions, but they involve polynomial operations like factoring.

  • When simplifying, always factor completely before canceling common factors. Remember that you can only cancel factors, not individual terms. For example, in \(\frac{a+b}{a}\), you cannot cancel \(a\).
  • When adding or subtracting, finding the LCD is crucial. Multiply the numerator and denominator of each fraction by the factors needed to make the denominator the LCD.
  • Pay attention to restrictions on the variables. The simplified expression is equivalent to the original expression only for values of the variables for which the original expression is defined (i.e., the denominators are non-zero). In this problem, \(c \neq 0\), \(a \neq c\), and \(a \neq -c\).
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